PROCESS TO RECOVER ALKALI FROM A METAL OXIDE/HYDROXIDE CONTAINING MATERIAL

Abstract

A process for recovering alkali from power boiler ash is provided. The power boiler ash is first contacted with Na.sub.2CO.sub.3 to produce a mixture containing settling and non-settling solid particles. A fraction of the settling particles is then separated from the mixture to produce a first clarified alkaline solution. The first clarified alkaline solution contains species such as NaOH and KOH depending upon the power boiler ash characteristics. The non-settling solid particles may optionally be further separated from the first clarified alkaline solution to obtain a second clarified alkaline solution. This process is also applicable for the extraction of alkali from other oxide/hydroxide containing materials.

Claims

1. A process for recovering alkali from power boiling ash materials comprising the steps of: contacting the power boiling ash materials containing a mixture of metals oxides and hydroxide with a Na.sub.2CO.sub.3 solution to obtain a slurry, wherein the slurry comprises settling solid particles, non-settling solid particles and alkali generated during the contacting step; and separating a fraction of the solid particles from the slurry to obtain a clarified alkaline solution.

2. The process of claim 1, wherein the power boiler ash further comprises CaO, MgO, Ca(OH).sub.2, or a combination thereof.

3. The process of claim 2, wherein the power boiler ash is fly ash.

4. The process of claim 1, wherein the power boiler ash is bottom ash.

5. The process of claim 1, wherein the power boiler ash is combined ash.

6. The process of claim 1, wherein the Na.sub.2CO.sub.3 is derived from waste or process liquor from an industrial plant.

7. The process of claim 1, wherein the Na.sub.2CO.sub.3 has a concentration of between about 5% and about 90% of the metal oxide/hydroxide containing material by dry weight.

8. The process of claim 1, wherein the concentration of alkali in the clarified alkaline solution is between about 2 g/L and about 85 g/L.

9. The process of claim 1, wherein an alkali yield defined as a mass of alkaline in g per 100 g mass of metal oxide/hydroxide containing material is of at least 2%.

10. The process of claim 9, wherein the alkali yield is of at least 35%.

11. The process of claim 1, further comprising separating a fraction of the non-settling solid particles from the clarified alkaline solution to obtain a second clarified alkaline solution.

12. The process of claim 11, wherein separating the fraction of non-settling solid particles comprises using a pulse filter, a membrane-based separation unit, a pressure filter, vacuum filter, filter press, a fabric filter, a centrifuge or any combination thereof.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0035] Reference will now be made to the accompanying drawings.

[0036] FIG. 1 shows a process for recovering alkali from power boiler ash in accordance in accordance to an embodiment.

[0037] FIG. 2 shows a plot of NaOH concentration as a function of Na.sub.2CO.sub.3 amount in the process of FIG. 1.

[0038] FIG. 3 shows a plot of alkali yield as a function of Na.sub.2CO.sub.3 amount, both pure and derived from green liquor, in the process of FIG. 1.

[0039] FIG. 4 shows a plot of alkali yield as a function of Na.sub.2CO.sub.3 amount, in a process for recovering alkali from cement in accordance with another embodiment.

[0040] It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION

[0041] In accordance with the present disclosure, a process for recovering alkali from a metal oxide (e.g., Na.sub.2O, K.sub.2O, CaO and MgO) or metal hydroxide containing material is provided.

[0042] It is provided process for recovering alkali from power boiling ash, comprising the steps of contacting the power boiling ash with Na.sub.2CO.sub.3 to obtain a mixture, wherein the mixture comprises settling solid particles, non-settling solid particles and alkali generated during the contacting step; and separating a fraction of the solid particles from the mixture to obtain a clarified alkaline solution.

[0043] The process provided herein applies universally to a range of oxides of metals such as MgO and thus to a variety of materials and not just to ashes resulting from the combustion of biomass or the chemical recovery process. As an example, the process encompassed herein applies to alkali production from cement.

[0044] In one non-limiting embodiment, with reference to FIG. 1, a process 100 for recovering alkali (polished alkaline solution 110) from a power boiler ash is shown. In a first step 102, the powder or slurried boiler ash 101 is contacted with Na.sub.2CO.sub.3, Na.sub.2CO.sub.3 being preferably in solution or solubilized form, to form a mixture. In an embodiment, the process 100 can comprise a preliminary leaching step with water to remove water-soluble impurities. The power boiler ash may be fly, bottom or combined ash. The term fly ash refers to the portion of ash that escapes the combustion zone with flue gas. The bottom ash refers to the heavier ash particles collected at the bottom of the boiler. The combined ash refers to a situation where the fly ash and bottom ash after generation are combined before final disposal. In one non-limiting example, the power boiler ash contains oxides/hydroxides of metals (such as but not limited to Na.sub.2O, CaO, MgO and Ca(OH).sub.2), which are sources of hydroxide alkalinity (i.e., of hydroxyl ions OH), as well as some carbonate and other anion-based species. In other non-limiting embodiments, the process 100 may be used on any other metal oxide/hydroxide containing material, such as but not limited to any suitable industrial material or process by-product including cement, biomass and the likes.

[0045] It is appreciated that, when Na.sub.2CO.sub.3 in solubilized form is used, at least a fraction of water-soluble impurities present in the power boiler ash may also be removed, as further described below. The Na.sub.2CO.sub.3 used at step 102 may be any commercially-available, pure Na.sub.2CO.sub.3 or Na.sub.2CO.sub.3 from green liquor (GL) from a chemical pulp mill. In the latter case, alkali recovery according to the process 100 may or may not be integrated as part of the chemical recovery cycle of a kraft mill. In this embodiment, Na.sub.2CO.sub.3 may be present during step 102 at a concentration of between about 5% and about 90% of the power boiler ash by dry weight depending on the level of oxides/hydroxides present in the ash. However, in most situations a sodium carbonate to ash ration of 20-60% would suffice.

[0046] The oxides/hydroxides present in the power boiler ash can react with water and/or the Na.sub.2CO.sub.3 in the mixture as shown in Equations 1-5 below.

MgO+H.sub.2O+Na.sub.2CO.sub.3.fwdarw.MgCO.sub.3+2NaOH   (Equation 1)

CaO+H.sub.2O+Na.sub.2CO.sub.3.fwdarw.CaCO.sub.3+2NaOH   (Equation 2)

Ca(OH).sub.2+Na.sub.2CO.sub.3.fwdarw.CaCO.sub.3+2NaOH   (Equation 3)

Na.sub.2O+H.sub.2.fwdarw.2NaOH   (Equation 4)

K.sub.2O+H.sub.2O.fwdarw.2KOH   (Equation 5)

[0047] In an embodiment, Na.sub.2CO.sub.3 reacts with metal (both calcium and non calcium-based) oxides present in the power boiler ash to form solid particles and alkali (in hydroxide form) in the mixture. The solid particles produced are mostly carbonate compounds, for example MgCO.sub.3 and/or CaCO.sub.3. As further described below, the solid particles in the mixture may be settling or non-settling (i.e., suspended, colloidal and/or dissolved). In the non-limiting example in which Na.sub.2CO.sub.3 is in solution: (i) Na.sub.2O and K.sub.2O present in the power boiler ash produce NaOH and KOH, respectively, by reacting with water according to Equations (4) and (5) above; and (ii) other metal oxides such as MgO, CaO and Ca(OH).sub.2 produce NaOH by reacting with Na.sub.2CO.sub.3 according to Equations (1), (2) and (3) above. Any other suitable alkali may be formed in other non-limiting examples.

[0048] The first step 102 may be performed in any suitable reaction tank. To increase the kinetics of the reaction(s) in the reaction tank, agitation, sonication or heating, may be used during the first step 102.

[0049] In a second optional step 104, a fraction of the settling solid particles generated during the contacting step 102 is separated from the mixture to form a first clarified alkaline solution. In this embodiment, the fraction of the settling solid particles settles at the bottom of the reaction tank such that they may be separated from the mixture in the reaction tank. In one non-limiting example, the fraction of the settling solid particles that settles at the bottom of the reaction tank may be recovered in the form of a residual slurry 103. The resulting residual slurry has high calcium carbonate content and can be used in construction, in agriculture and as a neutralizing agent (e.g. for pH adjustment). It is appreciated that, in this embodiment, the first clarified alkaline solution is therefore depleted from the fraction of the settling solid particles generated during the contacting step 102.

[0050] The first clarified alkaline solution that is depleted from the fraction of the settable solid particles generated during the contacting step 102 may exhibit levels of residual solid particles of between 0.1% and 10%, the residual solid particles comprising both the non-settling solid particles as well as settable solid particles that were not separated from the mixture at the second step 104. When the level of residual suspended solid particles is below 0.01%, the first clarified alkaline solution may be used directly, for example in applications such as make-up caustic, bleaching and neutralization agent as well as total reduced sulphur (TRS) scrubber solution. Still in this embodiment, the first clarified solution has a concentration of alkali (NaOH) in solution of between about 2 g/L and about 38 g/L. In an embodiment, the caustic solution generated has a concentration of between 4-10%. It is appreciated that when purchased Na.sub.2CO.sub.3 or that derived from GL is used, the desired alkaline (NaOH) concentration may be controlled via the fly ash to liquor (or water) ratio. With appropriate process conditions an alkali concentration of 10% (100 g/L) or even higher can be obtained. Still in this embodiment, the first clarified alkaline solution has an alkali yield (i.e., a mass of NaOH and/or KOH in g per 100 g dry power boiler ash) of 5 to 40%, preferably of at least 10%, in some cases at least 20%, in some cases at least 30%, in some cases at least 35%, in some cases at least 40% and in some cases even more. In an embodiment, the settling step 104 is skipped and the mixture sent directly to a solid/liquid separation 106 device such as a pressure, vacuum or a fabric filter (see FIG. 1). Another option is to send the mixture to the sewer of a plant where the use of lime is required to neutralize acids. The direct release of the solids from 102 will serve as an alkali and will neutralize the acids.

[0051] In step 106, the first clarified alkaline solution, depleted in the fraction of the settleable solid particles generated during the contacting step 102 or contributed by the ash, may be subjected to a further separation step in which a fraction or all of the settleable or non-settleable (i.e., suspended, colloidal and/or dissolved) solid particles and ions is separated from the first clarified alkaline solution or the slurry flowing directly from 102 to 106 to obtain a (second) clarified alkaline solution. In this embodiment, the step 106 may be performed by using a physical separation method, such as for example a membrane-based separation unit, a centrifuge, a pressure filter, a vacuum filter, a belt press or any other suitable separation technique in other embodiments. The clarified alkaline solution from 106 is therefore substantially depleted of solid particles generated during the contacting step 102 or contributed by ash. The clarified alkaline solution from 106 may then be stored in a tank for on-site consumption or subsequent shipment, the clarified alkaline solution being used for example in applications such as neutralization, bleaching, use in scrubbers, as caustic make up, as a solution to regenerate demineralization resins and membranes. In other non-limiting embodiments, the clarified alkaline solution from 106 may optionally be further purified or concentrated via reverse osmosis, nanofiltration or ultrafiltration or any other suitable process.

[0052] It is appreciated that the process 100 may be applied in the context of any mill (including kraft mills and mechanical pulp mills) or any biomass (or coal) fired cogeneration power plant. Some mechanical pulp mills such as closed cycle bleached chemi-thermomechanical pulp (BCTMP) mills produce a waste inorganic stream, after burning their heavy liquor, which is rich in sodium carbonate. Presently, this stream is landfilled. These types of pulp mills are not equipped with a causticizing plant to make use of the Na.sub.2CO.sub.3 and produce alkali (caustic). This Na.sub.2CO.sub.3-rich stream can be dissolved in water and used to recover caustic from ash. The NaOH can be employed for example in the bleach plant. The caustic stream can be purified if needed to remove any undesirable species, non process elements (NPEs) that may affect bleach plant operations.

[0053] The product obtained from the process described herein is clean enough to be used in applications such as bleaching, as make up caustic, as scrubbing solution for total reduced sulfur (TRS) removal and as a neutralization agent. However, if the recovered product needs refinement, an additional optional step 108 can be included to remove colloidal or dissolved species (NPEs) in the recovered alkali 110. Technologies to do so include but are not limited to membranes processes, ion exchange resins, surface adsorption and evaporation or a combination of them.

EXAMPLE I

[0054] With further reference to FIG. 2, data relating to the alkali recovery from a first power boiler ash sample is shown. Alkali leaches out of the first power boiler fly ash sample when it is solubilized in water (i.e., about 2 g/L with no Na.sub.2CO.sub.3). The alkali (i.e., NaOH) concentration increases as the amount of Na.sub.2CO.sub.3 added to the power boiler ash increases. As further shown in FIG. 2, the increase in NaOH concentration is steep at lower Na.sub.2CO.sub.3 quantities (e.g., between about 0% and about 40% of dry power boiler ash by weight) however it levels off possibly with the depletion of reacting metal oxides and hydroxides at higher Na.sub.2CO.sub.3 quantities (e.g., between about 40% and about 100% of dry power boiler ash by weight) or due to other process constraints such as kinetics and thermodynamics.

[0055] With further reference to FIG. 3, data relating to the alkali recovery from a second power boiler ash sample is shown using pure Na.sub.2CO.sub.3 and Na.sub.2CO.sub.3 present in GL or any similar stream from the pulp and paper or another industry. Much like in FIG. 2 above, the alkali yield increases with the Na.sub.2CO.sub.3 quantities. The alkali yield is also higher with the Na.sub.2CO.sub.3 present in GL compared to the pure Na.sub.2CO.sub.3, likely because at least some residual caustic is present in GL.

[0056] Table 3 below shows the quality (i.e. chemical composition) of the recovered alkali as a function of the Na.sub.2CO.sub.3 quantities. The alkali yield increases as the Na.sub.2CO.sub.3 quantities are increased. The quality of the recovered alkali varied with the Na.sub.2CO.sub.3 quantities. Most of the impurities were removed in the process and measured below detection (BD). Some sulfur was found to be present in the recovered product which, if used to maintain alkali/sulphur balance, would create additional value. For example, kraft pulp mills purchase sodium sulfate as a make-up up chemical. The caustic provided herein can be used in the recovery cycle, wherein less sodium sulfate will have to be purchased to maintain the sodium sulfur balance. Further treatment or purification of the produced alkali solution can be performed if desired using ion exchange or membrane filtration or other separation approaches. For example, impurities such as chloride and potassium (and metals) can be removed using ion exchange technology. Water may also be used to leach soluble species out of the power boiling ash before reacting it with sodium carbonate.

TABLE-US-00003 TABLE 3 Chemical composition of recovered alkali as a function of Na.sub.2CO.sub.3 quantities Na.sub.2CO.sub.3 to ash ratio, % 0 10 20 30 50 Elements mg/L mg/L mg/L mg/L mg/L NaOH* 2119   7822 17292 25231 31400 Al 0 BD.sup.3 BD.sup.3 781 2121 As BD BD.sup.3 BD.sup.3 BD.sup.3 6 B BD BD.sup.3 12 11 17 Ba 1 BD.sup.1 BD.sup.1 BD.sup.1 BD.sup.1 Ca 1912   460 BD.sup.4 BD.sup.4 BD.sup.4 Cd BD BD.sup.1 BD.sup.1 BD.sup.1 BD.sup.1 Co BD BD.sup.1 BD.sup.1 BD.sup.1 BD.sup.1 Cr 1 3 4 4 6 Cu BD 0.3 BD.sup.1 1 1 Fe BD 1 1 1 9 K 759  723 926 846 912 Li 0 BD.sup.3 BD.sup.3 BD.sup.3 BD.sup.3 Mg 0 BD.sup.1 BD.sup.1 BD.sup.1 BD.sup.1 Mn BD BD.sup.1 BD.sup.1 BD.sup.1 BD.sup.1 Mo   0.4 1 2 1 2 Na 366  7139 14400 20270 42900 Ni BD BD.sup.1 BD.sup.1 BD.sup.1 BD.sup.1 P BD BD.sup.2 BD.sup.2 3 30 Pb BD BD.sup.3 BD.sup.3 7 14 S 473  3018 4466 5718 6314 Sb BD BD.sup.1 BD.sup.1 1 2 Se BD BD.sup.2 BD.sup.2 BD.sup.2 BD.sup.2 Si BD BD.sup.3 12 65 368 Sr 14  7 2 0.3 BD.sup.1 Ti BD BD.sup.1 BD.sup.1 BD.sup.1 BD.sup.1 V BD BD.sup.1 0.4 0.3 2 Zn 1 3 12 44 127 Cl.sup.− 1329 1279 1363 1544 BD: below detection; MDL: method detection limit; BD.sup.1: MDL 0.005 ppm; BD.sup.2: MDL 0.01 ppm; BD.sup.3: MDL 0.1 ppm; BD.sup.4: MDL 1 ppm

[0057] With further reference to FIG. 4, data relating to the alkali recovery from Portland cement is also shown, with an alkali yield of at least 20%, in some cases at least 30%, in some cases at least 35% and in some cases even more with up to 100% Na.sub.2CO.sub.3.

[0058] While this disclosure has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations, including such departures from the present disclosure as come within known or customary practice within the art, and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.